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When Caltech biologist Michael Abrams cut two arms off a young jellyfish in 2013, he figured it would do what many marine invertebrates do—grow new ones. But no. "[Abrams] started yelling... 'You won't believe this, you've got to come here and see what's happening,'" his PhD adviser Lea Goentoro tells National Geographic. Reporting this week in the journal Proceedings of the National Academy of Sciences, Abrams says he watched the jellyfish, which relies on being symmetrical to move about, not regenerate the missing arms but rather rearrange its remaining six limbs so that they were symmetrical again. The phenomenon, dubbed symmetrization, has never before been observed in nature, and Abrams was floored.

The jellyfish was using its own muscles to push and pull on its remaining six arms to space them out evenly again. (They confirmed this by observing that muscle relaxants made the jellies unable to rearrange their arms, while increasing muscular pulses allowed them to rearrange their arms faster.) And the discovery was accidental; Abrams and his team had only been cutting into the common moon jellyfish to practice for their future study on what are called immortal jellyfish, which had yet to arrive in the lab. They've since observed symmetrization in moon jellies many times, and it takes anywhere from 12 hours to four days to complete. (Scientists recently made another staggering observation, this one in Norway.)